使用WebDriverWait類處理等待（sleep）的問題

用selenium進行web UI的自動化開發時，經常遇到loading需要等待的時候，或者需要驗證一個action之后某個dialog是否呈現或者消失。對於這類情況是不建議用sleep(xx)來死等，因為你無法准確判斷要等多久，少了就會報error，多了就會浪費整個腳本跑的時間。聰明的做法就是靈活等待，WebDriverWait（org.openqa.selenium.support.ui）這個類是很好的選擇。

/**
     * Wait until the element visible with customized timeout
     * @param driver
     * @param locator
     * @param errorMessage
     * @return
     */
    public WebElement waitForElementVisible(WebDriver driver,final By locator, long timeOutInSeconds, String errorMessage) {
        Function<WebDriver, WebElement> waitFn = new Function<WebDriver, WebElement>() {
            @Override
            public WebElement apply(WebDriver driver) {
                try {
                    WebElement el = driver.findElement(locator);
                    if (el.isDisplayed()) {
                        return el;
                    }
                } catch (Exception e) {
                }
                return null;
            }
        };

        WebDriverWait wait = new WebDriverWait(driver, timeOutInSeconds);
        if (errorMessage != "") {
            wait.withMessage(errorMessage);
        } else {
            wait.withMessage("Cannot find Element: " + locator.toString());
        }
        return wait.until(waitFn);
    } 

/**
     * Wait until the element present with customized timeout
     * @param driver
     * @param locator
     * @param errorMessage
     * @return
     */
    public Boolean waitForElementPresent(WebDriver driver,final By locator, long timeOutInSeconds, String errorMessage) {
        Function<WebDriver, Boolean> waitFn = new Function<WebDriver, Boolean>() {
            @Override
            public Boolean apply(WebDriver driver) {
                try {
                    driver.findElement(locator);
                    return true;
                } catch (Exception e) {
                    return false;
                }
            }
        };

        WebDriverWait wait = new WebDriverWait(driver, timeOutInSeconds);
        if (errorMessage != "") {
            wait.withMessage(errorMessage);
        } else {
            wait.withMessage("Cannot find Element: " + locator.toString());
        }
        return wait.until(waitFn);
    }